I have done extensive research on the overlap between .NET and Sybase ASE. Sybase released theirs ADO.NET 2.0 drivers for ASE 3 years after Oracle. Don t expect an Entity Framework provider anytime soon.
You could use Data Direct, but if your stakeholders are anything like ours, they are still sore about how much they spent on the latest round of license increases from Sybase. They aren t going to be excited about putting even more money into such an antiquated product.
The two ORM solutions currently available are LLBLGen Pro and NHibernate. Our project has successfully used NHibernate against Sybase 12.5 and 15. It works quite well for us. The current set of Sybase ASE drivers are a bit anemic, but they work just fine. We have supplemented these drivers a bit when we needed to. Tinkering with the drivers sounds hard, but its not that bad.
Update: For our purposes BLToolkit ended up being a better solution than NHibernate. Your mileage may vary.